1. Field of the Invention
The present invention relates to a planar antenna for receiving electric waves from a satellite for broadcasting or a satellite for communication. More particularly, the invention relates to a planar antenna suitable for receiving linearly polarized waves including vertical polarized waves and horizontal polarized waves.
2. Description of the Related Art
FIG. 4 is a top view of a conventional planar antenna, in which four antenna elements 32, 33, 34, and 35 for reception are arranged on the top face of a multilayer substrate 31 comprised of four conductive layers and three insulating layers. Each of the antenna elements 32, 33, 34, and 35 for reception is formed in a square shape by, for example, the conductive surface layer of the insulating substrate 31. The length of one side is set so as to be equal to about the half of the wavelength of a receiving wave. With the length, the resonance frequency of each of the antenna elements 32, 33, 34, and 35 coincides with the center frequency of the receiving wave.
The antenna elements 32, 33, 34, and 35 for reception are arranged in the vertical and lateral directions on the insulating substrate 31 in a state where one sides of neighboring elements face each other in parallel.
Between the facing two sides in the antenna elements 32, 33, 34, and 35 for reception, as illustrated in FIG. 4, a voltage Ev (hereinbelow, referred to as a vertical voltage) in the vertical direction based on vertically polarized waves and a voltage Eh (hereinbelow, called a horizontal voltage) in the lateral direction based on horizontally polarized waves are induced.
In order to separately take out the vertical voltage Ev and the horizontal voltage Eh induced by each of the antenna elements 32, 33, 34, and 35 for reception, an antenna element 36 for coupling is formed by a second conductive layer almost in the center of the antenna elements 32, 33, 34, and 35 for reception.
A part of the antenna element 36 for coupling and a part of each of the antenna elements 32, 33, 34, and 35 for reception are overlapped. In overlapped parts 37, 38, 39, and 40, the antenna element 36 for coupling is coupled to the antenna elements 32, 33, 34, and 35 for reception via a first insulating layer 31a of the multilayer substrate 31. As a result, the vertical voltages Ev and the horizontal voltages Eh induced by the antenna elements 32, 33, 34, and 35 for receptionare induced and synthesized by the antenna element 36 for coupling.
Transmission lines 41 and 42 coupled to the antenna element 36 for coupling, for separately taking out the vertical voltage Ev and the horizontal voltage Eh induced by the antenna element 36 for coupling are made by a third conductive layer so as to form an angle of 90 degrees. The transmission lines 41 and 42 are coupled to the antenna element 36 for coupling via a second insulating layer 31b. The transmission line 41 is provided in parallel to the direction of the induction of the horizontal voltage Eh. The transmission line 42 is provided in parallel to the direction of the induction of the vertical voltage Ev.
The horizontal voltage Eh is taken out from the transmission line 41 and the vertical voltage Ev is taken out from the transmission line 42.
Below the transmission lines 41 and 42, an earth conductive layer 31d as a lowermost layer is provided via a third insulating layer 31c.
Since the transmission lines 41 and 42 extend to the peripheral parts of the multilayer substrate 31, when terminals (not shown) connected to the transmission lines 41 and 42 are provided at ends of the multilayer substrate 31 by proper means, the horizontal voltage Eh and the vertical voltage Ev are easily taken out.
In the conventional planar antenna, however, the vertical and horizontal voltages induced by the antenna elements 32, 33, 34, and 35 are induced by the antenna element 36 for coupling via the insulating layer 31a and further connected from the antenna element 36 for coupling to the transmission lines 41 and 42 via the insulating layer 31b. There is, consequently, a problem such that a coupling loss is increased by a dielectric loss caused by the insulating layers 31a and 31b.
Further, since the conventional planar antenna is constructed by using the multilayer substrate 31, the structure is complicated and its fabrication method is accordingly complicated. Consequently, the price cannot be reduced.